I take on Uthgerd the Unbroken as a follower and give her Legendary Ebony Armor, Gauntlets, Boots, Sword, Bow, Shield and Dagger. I also give her a Circlet, necklace and ring. All of these items have two enchantments. I can see all this equipment on her. I take her to Lakeview Manor and make her my Steward. I leave to continue my Dragonborning and when I return Uthgerd is wearing her steel plate, or whatever it is. I ask her to follow me and see she still has all the equipment I gave her.

If I return to Lakeview and there is a Giant, bandits, wolves or a Dragon attacking my home she is in her original armor.

My question: Does she keep the benefits of the armor I've given her even though it is not showing?

A generic wife will keep stuff, but no matter what you give her, she won't wear/use it. House Carls or hired mercenaries not only keep the stuff but will use it. So give them really good stuff. Armor jewelry weapons all enhanced stuff you'll be tanks!